Method and device for cleaning out a well or piping blocked with gas hydrates

ABSTRACT

A method and apparatus for unplugging a well or pipe obstructed by a gas hydrates. According to the invention, a moving heating element is applied against one end of the hydrate plug, and is displaced axially in the tube or the pipe towards the other end, so as to cause the plug to melt progressively from one end to the other. Application to the oil industry, in particular in off-shore wells.

The present invention relates to a method and to apparatus forunplugging a well or a pipe which is obstructed by a solid plug of gashydrates.

BACKGROUND OF THE INVENTION

In the oil trade or industry, a problem sometimes arises associated withthe appearance of solid plugs inside a borehole or a pipe fortransporting oil products, in particular pipeline or a feed pipe.

In the present description, the terms “upstream” and “downstream” whenrelating to a well or pipe correspond to the direction in which the oilproduct flows in the well or in the pipe.

Thus, when referring to an oil well, for example, its upstream end islevel with the hydrocarbon reservoir, while its downstream end is at thesurface.

Hydrate plugs are observed to form in wells or pipes in which theinternal bore is at a pressure which is very high and at a temperaturewhich is low.

This occurs in particular in off-shore wells.

Hydrates are substances made up essentially of water, together withvarious chemical substances, and having a melting temperature of about0° C. at atmospheric pressure.

This melting temperature increases relatively fast with pressure. Thus,by way of indication, this melting temperature is about 10° C. at apressure of 20 bars, 15° C. at a pressure of 50 bars, 20° C. at apressure of 100 bars, and 22° C. at a pressure of 200 bars.

Still by way of indication, in the North Sea, where there are to befound numerous oil-bearing deposits in production, the temperature issubstantially constant and about 8° C. to 9° C.

The pressure that exists in the wells or pipes often lies in the range50 bars to 300 bars, and consequently corresponds to hydrates existingin the solid state.

To prevent such solidification, it is general practice to insertantifreeze liquids, in particular methanol, into wells or pipes for thepurpose of lowering the melting point of the mixture very significantly.

Although that method generally gives satisfaction, it can neverthelesshappen accidentally that solid hydrate plugs appear, thereby obstructingthe well or pipe and preventing it from being used.

So far as the Applicant is aware, there does not exist at present anymethod or apparatus that is really adapted to eliminating such plugs.

Two methods are used.

The first consists in expelling hydrate plugs by inserting a fluid undervery high pressure into the well or the pipe.

That solution is effective in certain cases only.

Another method consists in drilling through the plug(s), thereby makingit necessary to use equipment that is extremely sophisticated andexpensive, while nevertheless compromising the integrity of the well orthe pipe; there is a risk of the drilling tool damaging the walls of thewell or the pipe during the operation.

SUMMARY OF THE INVENTION

That is why the present invention proposes filling this void, byproposing a method and apparatus that enable the well or the pipe to beunplugged in a manner that is simple and of low cost, completely safe,and without running the risk of damaging the walls.

The method of the invention for unplugging a well or a pipe obstructedby a solid hydrate plug and which enables this object to be achieved isremarkable in that a moving heating element is pressed against one endof said plug, and is displaced axially in the tube or pipe towards theother end so as to cause the plug to melt progressively from one end tothe other.

In one possible implementation of this method, during the operation, an“antifreeze” liquid is supplied for lowering the melting temperature ofthe hydrate plug inside the well or the pipe in the vicinity of the endof the plug against which the heating element is applied.

Advantageously, the heating element is electrically heated.

According to another characteristic of the method, the heater element isdisplaced under the effect of gravity.

Naturally, this assumes that the well or the pipe extends in a verticaldirection or at least slopes with a significant vertical component.

In a variant, the element is displaced by thrust, with the thrust actionbeing assisted or otherwise by the effect of gravity.

In a preferred implementation of the invention, a moving heating elementis applied against one end of said plug while an antifreeze liquid iscaused to circulate inside the well or pipe in a closed circuit in thevicinity of said end while simultaneously displacing the heating elementaxially towards the other end of the plug in such a manner that the heatdelivered by the heating element and the presence of the antifreezeliquid together ensure that the plug melts progressively andirreversibly.

In which case, said heating element advantageously includes electricalresistances for providing heating by the Joule effect, while theantifreeze liquid is circulated by means of an electric pump, and theelectrical power for said electrical resistances and the pump isdelivered from surface equipment by means of conductors located in asuspension cable or rod.

The apparatus which also forms part of the present invention and whichserves to unplug a well or a pipe obstructed by a solid hydrate plug isremarkable in that it comprises a moving heating element adapted to beinserted in and displaced axially along the well or the pipe in such amanner as to be applied against one end of said plug and to cause it tomelt progressively from one end to the other while being displacedtowards the other end.

Furthermore, according to various additional but non-limitingcharacteristics of the invention:

said heating element is a metal body that conducts heat and that houseselectrical resistances that are electrically powered from surfaceequipment;

the apparatus includes means for injecting an antifreeze liquid into thewell or the pipe in the vicinity of the hydrate plug;

the apparatus includes means for continuously recycling the antifreezeliquid;

said heating element is an elongate body whose free end for beingapplied against the hydrate plug is of generally tapering shape; and

the apparatus is suspended from a cable connected to the surfaceequipment, and it is lowered by gravity inside the tube or the pipe,said cable containing various electrical conductors enabling theapparatus to operate, in particular to power the heating resistancesand, where appropriate, the pump which circulates the antifreeze.

In a preferred embodiment, the apparatus comprises:

a) a moving heating element constituted by an elongate metal body thatconducts heat, the free end of the body for application against thehydrate plug being of a generally tapering shape;

b) electrical resistances housed in the body and suitable for heating itby the Joule effect;

c) an electric pump secured to the heating element and adapted to causean antifreeze liquid to circulate in a closed circuit around thetapering end; and

d) a cable or rod to which the heating element and the pump are fixedand via which they are connected to surface equipment, said cable or rodcontaining electrical conductors for electrically powering theresistances and the pump.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention appear from thedescription and the accompanying drawings which show one possibleembodiment by way of non-limiting example.

In the drawings:

FIG. 1 is a diagrammatic overall view of the apparatus installed on site(in a well).

FIG. 2 is a diagrammatic view of apparatus of the invention shownpartially in section on a vertical axial plane.

FIG. 3 is an axial section view of the heater element constituting partof the apparatus.

FIG. 4 is a cross-section view on a larger scale through the element ofFIG. 3, the section plane being referenced IV—IV in FIG. 3.

FIG. 5 is a view similar to FIG. 2 on a smaller scale and showing theunplugging operation.

FIGS. 6 and 7 are diagrammatic views of two variants of the apparatusinside a pipe.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1, there can be seen a diagram of an off-shore oil well whosedrilling platform PF supports surface equipment ES of conventional typeand including, amongst other things, a winch T and a telemetry moduleMT.

The well has a casing assembly designated by the reference C.

In general, such an assembly is made up of three concentric tubes knownin the art (from the outside towards the inside) by the terms “riser”,“casing”, and “tubing”.

Only the downstream end portion of this casing is shown, which portionin the example shown is partially immersed in a liquid L, specificallyin sea water.

By way of indication, the immersed length of the pipe C is of the orderof several tens to several hundreds of meters, while the portion in theground and giving access to the hydrocarbon reservoir is much longer.

Except in the relatively shallow surface layer where temperaturefluctuates, the temperature of the mass of water is substantiallyconstant. As mentioned above, in the North Sea, this temperature isabout 8° C. to 9° C.

It is in this zone that the problem of hydrate plug formation arises.

Hydrates are generally in the form of isolated plugs of unit lengthlying in the range 1 meter to 10 meters, with plugs being separated byliquid zones. In all, the entire plug can reach a length of severalhundreds of meters.

The apparatus 1 of the invention essentially comprises a head 2, a tool3, and a suspension cable or rod 4.

The cable or rod 4 whose free end is attached and connected to the tool3 can be of the type that is generally used for logging, and comprises abundle of electrically conductive cables suitable for delivering theelectricity required for powering and controlling the equipment lowereddown the well from the surface equipment, and also suitable fortransferring to the surface, i.e. in the opposite direction, the variouselectrical signals that emanate from the various sensors with which theequipment is fitted.

The cable or rod 4 is wound on the drum of the winch T.

The module MT controls the winding-out or -in of the cable or the rod bymeans of said drum, in such a manner as to drive the apparatus 1 down orup before, during, and after the operation.

The module MT is also electrically connected to the cable or rod 4 inorder to perform the operation, as explained below.

In FIGS. 1 and 5, there is shown the top portion of a solid hydrate plugthat has formed accidentally in the casing C.

This constitutes a cylindrical plug of ice given reference H in thefigures.

The head 2 is substantially bullet-shaped having a cylindrical bodywhose downwardly-directed free end or nose is of tapering shape, e.g.being frustoconical.

By way of indication, if the inside diameter of the casing C is 150 mm,the diameter of the head 2 is about 90 mm.

Its height, referenced l₂ in FIG. 2, is about 1 meter, for example.

The tool 3 in the form of a cylindrical rod is fixed to the head 2 byappropriate means. Its diameter is significantly smaller than that ofthe head 2.

By way of indication, the total length l₁ of the apparatus is about 5meters.

As explained below with reference to FIGS. 3 and 4, the portion 2 is aheater element of a material that is a good conductor of heat. Thematerial constituting it also has good mechanical strength properties.

Various metals possess the required characteristics; as examples,mention can be made of steel and bronze.

In its “downstream” end portion (remote from the head 2), the rod 3 isfitted with an electric pump 8 associated with a strainer 5.

The pump is adapted to suck in the surrounding liquid that is to befound around the strainer and to deliver it into a central tube 6 on theaxis of the rod 3.

The tube 6 opens out into a central channel 20 passing through theheater element 2 from one end to the other.

As can be seen in FIGS. 3 and 4, the heater element 2 is provided withelectrical heating resistances 7. In the example shown, there are sixsuch resistances regularly distributed at 60° intervals around thecentral channel 20.

These resistances are of conventional type, e.g. of the type referred toas “heating pencils” of composite steel/ceramic structure having Jouleeffect heating wires integrated therein.

Advantageously, the periphery of the element 2 is fluted by a set oflongitudinal grooves 22, of which there are likewise six, extending inregister with the gaps between pairs of adjacent heating resistances(see FIG. 4).

Reference 200 designates the outlet from the channel 20 which opens outaxially into the central portion of the nose 21.

The heating elements 7 and the electric pump 8 are electrically poweredby appropriate conductors (not shown) which are grouped together in thecable or the rod 4 and which communicate with the surface equipment ES.

The apparatus 1 also has a set of electronic sensors (not shown) ofconventional type that are likewise connected to the surface equipmentand that serve to measure and monitor various parameters during theoperation, in particular temperature, antifreeze flow rate, and thepressure with which the head 2 is applied against the hydrate plug H.

That constitutes instrumentation that is usual in the field of drillingand managing an oil well, which is why it is not described in greaterdetail herein so as to avoid pointlessly burdening the description.

There follows a description of how a solid hydrate plug H is eliminatedusing the apparatus as described above.

A casing C is generally filled with a liquid mixture of water andantifreeze. As an appropriate antifreeze, mention can be made ofmethanol.

Preferably, prior to the operation, a certain volume of relativelyundiluted antifreeze is injected using appropriate and conventionalmeans into the inside of the casing C into the zone situated immediatelyabove the solid plug H.

The apparatus is lowered inside the casing C by actuating the winch T inthe winding-out direction, as represented by arrow X.

This displacement is stopped when the head 2 comes to bear against thetop (or downstream) end EA of the plug H.

Pressure sensors (not shown) mounted on the apparatus 1 serve toidentify this situation, and consequently to cause movement to bestopped and restarted via the module MT.

Electrical power is fed to the heating resistances 7 and to the pump 8.

By way of indication, the heating elements 7 are powered from thesurface using DC, at a voltage lying in the range 300 V to 1000 V, andat power lying in the range 5 kW to 15 kW, e.g. about 7 kW to 8 kW.

The temperature of the element 2 is advantageously raised to a valuelying in the range 50° C. to 100° C.

The delivery rate of the pump 8 can be a few liters per minute.

Because of its contact with the portion 21 of the heated head 2, andalso because the ambient liquids are heated at this level, the end EA ofthe hydrate plug begins to melt.

The tapering shape of the heating head facilitates penetration thereofinto the plug, causing the plug to melt progressively.

As melting takes place, the apparatus 1 moves down inside the tube underthe effect of gravity, with the winch T being controlled appropriatelyto allow the cable 4 to “run out” and/or by applying axial thrust when arod is used (i.e. a rod that is axially rigid).

In addition, with the pump 8 in operation, the mass of liquid 9 situatedabove the plug H and containing antifreeze is put into circulation.

The circulatory path followed by the liquid is represented by arrows inFIG. 5.

The pump 8 delivers the antifreeze mixture downwards into the pipe 6 andthen into the channel 20 (arrows f).

At the outlet 200 from the element 2, the mixture comes into contactwith the surface EA of the plug (arrows i).

This has the effect of opposing re-solidification of the plug and ofmaking the melting thereof irreversible.

The mixture then rises all around the element 2, including within itsperipheral grooves 22 and then along the tool 3 (arrows j).

Thereafter some of the liquid is sucked in through the strainer 5(arrows k).

In order to homogenize the liquid mass situated above the plug H, whichmass also includes the hydrates that have just been melted, it ispreferable to displace the apparatus 1 periodically with reciprocatingvertical back and forth motion, as symbolized by double-headed arrow Y.This motion is controlled by the module MT.

It is commonplace for a single casing to be obstructed by a plurality ofhydrate plugs separated by liquid zones.

In accordance with the invention, it is naturally the first plug whichis eliminated first by melting, after which the apparatus is displaceduntil it comes to bear against the following plug, and the operation isrepeated successively for each of the plugs.

By way of indication, the rate of progress of the heating head inside ahydrate plug while it is being melted is about 1 meter per hour.

The combination of melting and circulating an antifreeze mixture makesit possible to work in optimum manner.

Providing the element 2 possesses a large amount of thermal inertia, itcan be heated intermittently (discontinuous electrical power) instead ofbeing powered permanently throughout the operation. In which case,antifreeze pumping can also be discontinuous, e.g. having periods ofheating alternating with periods of pumping.

In FIG. 6, the method is implemented in a pipe C, e.g. a horizontalpipe. The device 1 is fixed to a flexible rod 4′ which is axially rigid,but which possesses a certain amount of flexibility enabling it tofollow any curves that might exist in the pipe.

The rod 4′ is engaged in the tube, and it is pressed against the hydrateplug H by applying thrust X to the rod by appropriate means (not shown)situated at its other end, outside the pipe.

In the variant of FIG. 7, the device 1 is carried by a small carriage 10connected to the cable 4. The carriage is provided with guide wheels 11bearing against the inside wall of the pipe C. At least some of thewheels are drive wheels, with the directions of rotation thereof beingrepresented by arrows in the figure. They are driven by an electricmotor which is likewise powered via the cable 4. When the carriage is inoperation, it displaces the heating head 2 inside the pipe C so as topress it against the hydrate plug H, and push it against the plug so asto enable the operation to be performed.

What is claimed is:
 1. A method of unplugging a well obstructed by asolid hydrate plug, wherein a moving heating element is pressed againstone end of the plug, and is displaced axially in the tube or pipetowards the other end of the plug so as to cause the plug to meltprogressively from the one end to the other end of the plug; and duringthis operation, an antifreeze liquid is supplied for lowering themelting temperature of the hydrate plug inside the well in the vicinityof the end of the plug against which the heating element is applied. 2.A method as claimed in claim 1, wherein the heating element iselectrically heated.
 3. A method as claimed in claim 1, wherein theheating element is displaced under the effect of gravity.
 4. A method asclaimed in claim 1, wherein the heating element is displaced under theeffect of thrust.
 5. A method of unplugging a well obstructed by solidhydrate plug, wherein a moving heating element is applied against oneend of the plug while an antifreeze liquid is caused to circulate insidethe well in a closed circuit in the vicinity of the one end of the plugwhile simultaneously displacing the heating element axially towards theother end of the plug in such a manner that the heat delivered by theheating element and the presence of the antifreeze liquid togetherensure that the plug melts progressively and irreversibly.
 6. A methodas claimed in claim 5, in which the heating element includes electricalresistances for providing heating by the Joule effect while theantifreeze liquid is circulated by means of an electric pump, whereinelectrical power for the electrical resistances and the pump isdelivered from surface equipment by means of conductors located in asuspension cable or rod.
 7. Apparatus for unplugging a well obstructedby a solid hydrate plug, the apparatus comprising: (i) a moving heatingelement adapted to be inserted in and displaced axially along the wellin such a manner as to be applied against one end of the plug and tocause it to melt progressively from the one end to the other while beingdisplaced towards the other end of the plug; (ii) means for injecting anantifreeze liquid into the well or the pipe in the vicinity of thehydrate plug; and (iii) means for continuously recycling the antifreezeliquid.
 8. Apparatus for unplugging a well obstructed by a solid hydrateplug, the apparatus comprising a moving heating element adapted to beinserted in and displaced axially along the well in such a manner as tobe applied against one end of the plug and to cause it to meltprogressively from the one end to the other while being displacedtowards the other end of the plug; the apparatus being fixed to a cableor rod connected to the surface equipment and displaced by gravityinside the well, the cable or rod containing electrical conductorsenabling the apparatus to operate to power heating resistances in theheating element.
 9. Apparatus for unplugging a well obstructed by asolid hydrate plug, the apparatus comprising a moving heating elementadapted to be inserted in and displaced axially along the well in such amanner as to be applied against one end of the plug and to cause it tomelt progressively from the one end to the other while being displacedtowards the other end of the plug; the apparatus being fixed to a cableor rod connected to the surface equipment and displaced by gravityinside the well, the cable or rod containing electrical conductorsenabling the apparatus to operate to power a pump which circulatesantifreeze in the well.
 10. Apparatus for unplugging a well obstructedby a solid hydrate plug, the apparatus comprising: a) a moving heatingelement constituted by an elongate metal body that conducts heat, thebody having a free end for application against the hydrate plug of agenerally tapering shape; b) electrical resistances housed in the bodyand suitable for heating it by the Joule effect; c) an electric pumpsecured to the heating element and adapted to cause an antifreeze liquidto circulate in a closed circuit around the tapering end; and d) a cableor rod to which the heating element and the pump are fixed and via whichthey are connected to surface equipment, said cable or rod containingelectrical conductors for electrically powering the resistances and thepump.